This invention relates generally to low expansion austenitic alloys and more particularly to an alloy and an article formed therefrom for precision optical or other applications requiring dimensional stability at temperatures below -30 C. (-22 F.).
Invar, a low expansion alloy containing nominally 36% by weight nickel--balance iron plus minute quantities of manganese, silicon, and carbon amounting to less than 1 w/o, has a relatively low and exceptionally flat thermal expansion characteristic from about room temperature up to about 200 C. (392 F.). The expansivity of Invar in that temperature range is approximately one-tenth that of stainless steel. For example, "Carpenter Invar 36", a registered trademark for an alloy produced by Carpenter Technology, Inc. of Reading, Pa., assignee of this application, has a mean coefficient of thermal expansion of about 1.times.10.sup.-6 /.degree.C. to 2.times.10.sup.-6 /.degree.C. over the temperature range -18 C. to +175 C. (-0.4 F. to +346 F.). Due to its relatively low expansion behavior Invar is often used in such applications as precision optical devices, microscopes, for example, where even small dimensional changes due to temperature fluctuation cannot be tolerated. Invar has also been used in connection with a high expansion alloy to form bimetallic members such as are used in thermostats or the like.
In certain very high precision optical applications, such as ring laser gyroscopes, which are used in environments where extreme subzero temperatures are encountered, it is highly desirable that the low, flat expansion characteristic of Invar be exhibited at temperatures down to about -50 C. or lower. Moreover, due to the extremely low expansivity of the optical glass used in such devices it is critical that the coefficient of thermal expansion of the Invar be very small in order to minimize expansion mismatch between the metal and the glass.
Brace, U.S. Pat. No. 1,689,814 relates to an Invar type alloy containing 1-20% by weight colbalt in which the iron to nickel ratio may vary between 3:1 and 1:1. According to Brace, the addition of the cobalt lowers the coefficient of thermal expansion in the range from room temperature to 300 C. Brace also indicates that the inflection point, i.e. the inception temperature of the ferromagnetic to paramagnetic transition, of Invar is shifted to a lower temperature by decreasing the nickel content while increasing the iron and cobalt content of the alloy.
H. Masumoto, "On the Thermal Expansion of the Alloys of Iron, Nickel and Cobalt, and the Cause of the Small Expansibility of Alloys of the Invar Type", Science Reports, 1st Ser., Vol. 20, Tohaku University Research Institute (1931) relates to nickel-cobalt-iron alloys of the Invar type containing less than 10% cobalt. These alloys are designated by Masumoto as Super Invars because, according to Masumoto the addition of even small amounts of cobalt significantly reduces the coefficient of thermal expansion of conventional Invar.
Super Invar having a nominal composition of 31% nickel--5% cobalt--balance iron exhibits a coefficient of thermal expansion which is less than one-half that of regular Invar. For example, the Super Invar alloy produced by Carpenter Technology, Inc. has a maximum coefficient of thermal expansion of 0.63.times.10.sup.-6 /.degree.C. from -17.8 C. to 93 C. (0 F. to 200 F.).
Although Super Invar has a substantially lower expansivity than Invar, it is susceptible to martensite transformation in the vicinity of -30 C. (-22 F.), due to its reduced nickel content. It is well known that the austenite to martensite transformation results in a sudden expansion of the metal. In precision optical applications such marked expansion can be catastrophic by causing damage to precision optical glass devices which normally have extremely small expansivities at temperatures substantially below room temperature, for example less than -50 C. (-58 F.).
It is highly desirable, therefore, to have a Super Invar which not only has a very low coefficient of thermal expansion at temperatures from significantly below -50 C. (-58 F.) up to about 100 C. (212 F.), but which also exhibits thermal stability, that is, a martensitic transformation inception point which is controlled so as to fall well below the lowest temperature to be encountered in the particular application. Articles fabricated with such an alloy in close relation to another low expansion material such as glass would be essentially free of risk of damage from expansion mismatch due to the volumetric change which accompanies the martensite transformation.